Electrodynamic device



I April 2, 1929. A. L. THURAS v v1,707,544

' Y EC I Filed Aug 4, 1926 /o v ,5 W

Alber/ L. hum! I structure of the present Patented Apr. 2 1929.,

ALBERT L. THURAS, 0E EA LABORATORIES, INCORI YORK.

- UNITED STATES.

PATENT OFFICE.

Y., A CORPORATION 01' NEW ELEGTRODYNAMIC DEVICE.

' Application filed August 4, 1926. Serial No. 127,152..

This invention relates to electrodynamic devices and particularly to an acoustic devlce employing an electrodynamically actuated vibrating element.

An object of the invention is to increase the operating efficiency of electrodynamic devices. A more specific object is to convert elecinto acoustic energy with high and substantially uniform efliciency over a wide frequency range.

' In carrying out the invention there is pref erably prov ded a light-piston type phragm which is diadriven by'a light rigid coil secured to the diaphragm near the periphery of its piston portion.

The mass of the coil is made approximately equal to the effective massof the diaphragm in order to' obtain a maximum operating efliciency in the vibrating' structure. diaphragm of the mately equal to the tion.

1 In its preferred The effective mass of a piston type is approximass of its vibrating p0remploys a diaphragm and a sound chamber in accordance withthe structure disclosed and claimed in a copending application of E. G. Wente, Serial No. .12 4, 1926, whereby the passage takes place 0 23,. filed Aug. cutofi point of the sound at a frequency above the essential range employed in speech and music. The low frequency cut-off point of a loud speaking system employing the vibrating determined by nentially tapered horn sage'approximately 11' having a sound pasm length 1n order to uniformly transmit frequencies down to 80 cycles. By increasing this cut-off frequency'may desired frequency.

The invention may by referring to the the length of the horn be lowered to any be readily understood accompanymg drawing in conjunction with the following detailed description.

In the drawing,

Fig. 1 is a sectional view of a loud speaker employing the present invention Fig. 2 is an enlarged view of a portion of the diaphragm and the coil attached thereto;

. through t invention is largelyv the length and mouth opening Fig. 3 is a view taken along the line 3-3 of Fig. 1.

top portion 15 of the magsembling of thewinding 11 on the core structure', and is secured to the lower portion-of the core'by the screws 16. The diaphragm to which the coil 14 is attached comprises a stiff dish-shaped portion 17,-a flexible corrugated portion 18, and a flat portion 19 which .is clamped between the housing structure 20 and the upper portion 15' of the core structure;-

15 of the magnetic structure and the housing 20 by the clamping rings 21, and is clamped ]position 'by screws which pass e flanged portion of the housing 205'washers 21 and the flat portion 19 of the diaphragm and are threaded into the upper form, the present invention portion 15 of the magnetic structure.

The metallic plug 23 in the form of a spherical sector 1s secured to the housing 20' that the area of the soundpassage formed be- I tween these surfaces progressively increases from the center of the diaphragm and from its peripheral portion toward the sound passage formed by the housing 20 and the plug 23. The rate of increase is such as to maintain a constant velocity in the air chambers. The remainder of the sound passage is also preferably designed to progressively increase in area from the diaphragm to the mouth of the horn 26.

The space at the mid-portion of the core 10 is closed at each end by a material having a series acoustic resistance such as the fine mesh wire gauze members 27 and 28 and a sound absorbing material 29, such as wool is employed to fill the space between these gauze members. Such an arrangement substantially eliminates acoustic resonance at this portion of the structure.

Since the large gain in the operating .efiiciency of a receiver constructed in accordheld in a The diaphragm is separated from the portion 1 ance with this invention is due to the specific embodiment of the coil and the diaphragm described herein, the method of constructing the coil and the method of attaching the coil to the diaphragm is of considerable importance. While the relative magnitudes of the various factors entering into the construction of the vibrating element of the receiver are often of more importance than the specific values of individual magnitudes, yet it seems desirable, for the sake of clearness, to point out the specific magnitudes by the use of which desired results have been obtained.

The process of making the coil 14 consists in coating a clean aluminum ribbon, having a thickness of about .002" and a width of about .014, with a thin solution of a phenolic condensation product dissolved in acetone. The coated ribbon is then subjected to a temperature of about 350 C, for .a short period in order to thoroughly harden the coating. The coated conductor is then passed through a second solution of av phenolic condensation product, preferably slightly more viscous than the previous coating solution, after which it is subjected to a temperature of about 250 C. in order to cause the evaporation of some of the solvent. This leaves the coating in arelatively dry but unhardened condition The coated ribbon is then wound edgewise on a collapsible mandrel to form a coil about 2 in diameter which is baked at a temperature of about 250 C and gradually cooled to room r temperature. The coil, now rigid and substantially free from internal strains, is removed from the mandrel and trimmed down to about turns which gives the coil a height of about .105". The inner surface of the coil is coated with a cementing material for example, a phenolic condensation product, or shellac and one edge of astrip 30 of fabric material, such as oiled silk, is secured thereto by means of the cement. The coil is again placed on a mandrel, the diameter of which is less than that of the mandrel previously used by an amount equal to the thickness of the silk, and is heated to about 150 C. in order to thoroughly dry the cement after which the opposite edge of the strip of oiled silk is socured to the diaphragm near the periphery of the stifiened portion 17 and the end portions 35 and 36 of the ribbon conductor are insulated and secured along the troughs of the corrugations of the flexible portion 18 of the diaphragm by means of a coating of shellac- The assembled structure 1s again heated to a temperature of approximately 150 C. after which it is ready to be mounted in the receiver.

The coil is positioned midway between the annular pole pieces 12 and 13 which are sepa rated by about .035" and the ends of the rib" bo n conductor are secured to the screws 31 and 32 which are electrically connected to the terminals 33 and 34, respectively. Since the width of the coil and fabric is about .017 there remains a clearance of only about .009" betweeneither pole piece and the coil. By

employing the fiat conductor and the very thin layers of insulating and stiffening matemass in grams is approximately equal to 8.0..

Due to the presence of impurities, this value may be slightly increased, but should in no case be above 90. For high efiiciency, Em l-TC" over, it is necessary to employ a diaphragm the vibrating portion of which is sufliciently large to cause a large volume displacement When vibrating and in order to obtain a diaphragmhaving a stiffened portion of suiticient rigidity it is necesasry to employ material of a certain minimum thickness. Good results have been obtained by employing an about a maximum mechanical efficiency while the small air gap between the pole pieces of the magnet and the compact coil ,fillingthe total available space therebetween results in a maximum electrical efficiency It 18 also important to note that the centers of gravity of the coil and the diaphragm are close together which practically eliminates the tendency for the coil to vibrate independently of the diaphragm.

The flexible portion 18 of the diaphragm preferably consists of substantially straight corrugations extending perpendicular to radii drawn to the inner ends of the corrugations as described in a copending application of H. C. Harrison, Serial No. 102,454, i'iled April 16, 1926. The reactance due to the compliance of this portion of the diaphragm is preferably made about e ual in magnitude to the mass react-ance of: t e air which the diaphragm sets into vibration When a horn having no inherent losses is employed in conjunction with the receiver constructed in accordance with this invention, an efiiciency of about 30% has been obtained, measured from the electrical power input to the acoustic power output, this being a substantial gain over other receivers. V7 hen employing a horn having a sufliciently large mouth opening and a sufficient length, the efficiency of the device is substantially uniform over a frequency range from about 80 cycles to above 5000 cycles. While the invention is of special value in conjunction with an acoustic device as described, a driven means other than a diaphragm may be employed and this feature is to be included within the scope of the present invention.

What is claimed is:

1. In combination, a diaphragm of light material and a light stiffcylin'drical coil consisting of a fiat insulated conductor wound edgewise and connected to said diaphragm for driving it. v

2. In combination, a diaphragm having a stifi'ened central portion and a flexible portion surrounding said central portion, a

magnet, a coil of a thin rectangular shaped.

insulated conductor in operative alignment with said magnet and connectedto said diaphragm at the edge of its stiifened portion,

the product of the resistivity in microhms per cubic centimeter of the conducting material of said coil by the mass of said material in grams per cubic centimeter being less than 3. An electrodynamic receiver comprising a diaphragm, a casing therefor, 1 a magnet, and a coil consisting of a single helix of a flat insulated conductor wound edgewise and secured to said diaphragm for driving it, said coil being of such dimensions that it substantially fills the available space between the pole pieces of said magnet. 1 t

4. The process of making a vibrational elementfor an electrodynamic receiver comprising. a diaphragm and a coil, which consists in coating a fiat conductor with a thin layer of insulating material, baking said insulated conductor, coating said conductor with a second thin layer of insulating material, drying the second layer of insulating ma-' terial, winding said insulated conductor edgewise on a collapsible mandrel to form said coil,

baking said coil, removing said mandrel, ce-

Pltent No. 1,707,544.

- CERTIFICATE 0F CORRECTION. I

" ALBERT L. THURAS.

menting one edge of a fabric to said coil, and

cementing another edge of said fabric to said diaphragm.

5. An electrodynamic acoustic device comprising a diaphragm having a stiffened central portion and a flexible peripheral portion, and a coil attached to the outer edge of the central portion consisting of an edgewisewound rectangular shaped conductor whose convolutions are insulated from each other and held together by an insulating cement.

shaped conductor, an insulating cement for securing the convolutions of said coil together -and means for securing said coil to sai diaphragm between its center and rim, said coil .bein positioned in said air gap, the clearance etween said coil and the pole pieces being not greater than .009".

48. An electrodynamic device comprising a diaphragm composed mainly of aluminum comprising a flexible portion and a plurality of curved surfaces joined together to form a stiff portion'and a coil secured to the stiff portion of said diaphragm for driving it, comprising a ribbon-like conductor mainly of aluminum wound on and cemented to a piece of fabric. y

In witness whereof, I hereunto subscribe my name this 4 day of August, A; D. 1926.

ALBERT L. THURAS.

Granted-April 2,1929. to

lt is herebycert-ifiedthatl error appears in thcIpriiited oecitication .ot.

above numbered patent requiring correction as follows! Page 2, line 84. the

number "90" read "9.0"; and that the sai-d Letters Patent. should-be read-withthis correction therein that th the Patent Office. 30th day of April; A, D- 1929.

Signed and sealed this,

(See!) I 0 same. may conform to the record of the case iin' f LIL J, hloore, Acting Commissioner of Patents.

CERTIFICATE OF GGRREGTION.

Patent No. 1,707,544. Granted April 2, 1929, to

ALBERT L. THURAS.

it is hereby certified that error appears in the printed specification oirthe above numbered patent requiring correction as ioilows: Page 2, time 84, for thenumber "90" read "9.0"; and that the said Letters Patent should be read with this correction therein that the same may ceniorm to the record oi the case in the Patent Office.

Signed and sealed this 30th day of April, A. D. 1929.

M. J. Moore, (Seal) v Acting Commissioner of Patents. 

